This invention relates to an improvement in a dual spark plug internal combustion engine having two spark plugs in each combustion chamber thereof, which is provided with an EGR (Exhaust Gas Recirculation) system for recirculating a considerably large amount of exhaust gases back to the combustion chamber.
In connection with the control of nitrogen oxides (NOx) formed in automotive internal combustion engines, a dual spark plug internal combustion engine provided with an EGR system has been proposed as disclosed in the pending application of Yasuo Nakajima et al., U.S. Pat. Ser. No. 815,449, filed on July 13, 1977 and entitled "Multiple Spark Ignition Internal Combustion Engine with Exhaust Gas Recirculation", now abandoned. In such an engine, two spark plugs are located relatively separate from each other in each combustion chamber. They ignite the air-fuel mixture which is mixed with exhaust gases recirculated back into the engine intake air at a relatively high EGR rate (the volume rate of recirculated exhaust gases relative to engine intake air), for example, about 30%. By virtue of the dual spark plug ignition, fast burn is achieved in the combustion chamber even though the air-fuel mixture is mixed with a considerably large amount of inert exhaust gases. NOx formation is greatly lowered without degradation of engine stability, resulting in an improvement in the driveability of a motor vehicle on which the engine is mounted.
The engine of this type requires operation on a single spark plug ignition in which one of the two spark plugs in each combustion chamber is put into inoperative condition during high power output engine operating condition. If dual spark plug ignition is continued during high power output engine operation, combustion pressure in the combustion chamber is excessively raised, shortening the life of the engine and increasing engine noise. This excessive rise in combustion pressure results from improved combustion in the combustion chamber, which is caused by the fact that charging efficiency of the engine is improved and EGR rate is decreased to a great extent from view points of engine power output and fuel economy during high power output engine operation.
In view of the above, it has also been proposed, as disclosed in the pending application of Shigeo Muranaka et al, U.S. Pat. Ser. No. 787,211, filed on Apr. 13, 1977 and entitled "Improved Dual Spark Plug Ignition Engine", now U.S. Pat. No. 4,144,860, that one of two spark plugs in each combustion chamber is rendered inoperative upon sensing a low intake vacuum which represents the high power output engine operation. However, such an engine has encountered the problem in which dual spark plug ignition is not operable during engine starting even though dual spark plug ignition is necessary for effectively initiating ignition of the air-fuel mixture. This results from the fact that the intake vacuum is very low or close to atmospheric pressure because of very low engine speed during cranking by the starting motor.